Vacuum degassing process

ABSTRACT

A process for degassing a continuously flowing liquid stream is executed using an apparatus that comprises an air-tight housing, in which is mounted for rotation a centrifuge bowl of circular cross-section and having an inner surface for spreading centrifugally impelled liquid to be degassed and an annular peripheral trough for retaining centrifugally impelled degassed liquid. The liquid stream is fed into the bowl and the degassed liquid is withdrawn through a tube extending from an exit line outside the housing to an open end of elongate cross-section which is positioned within the trough of the bowl and is immersible in the degassed liquid. The pressure of the exiting degassed liquid is measured, the pressure measurement being used to regulate the flow of liquid into the bowl so as to maintain a substantially constant level of the liquid therein. Gas is withdrawn to maintain a sub-atmospheric pressure within the apparatus housing, and the flow of the degassed liquid stream is regulated so that the liquid level in the bowl remains substantially constant.

This is a division of application Ser. No. 07/820,449 filed on Jan. 13,1992, now U.S. Pat. No. 5,190,515.

FIELD OF THE INVENTION

The present invention relates to an apparatus for degassing liquids andmore particularly to an apparatus for degassing liquids atsub-atmospheric pressures, and to a process for degassing liquids.

BACKGROUND OF THE INVENTION

Many liquids used in industrial processes contain entrained and/ordissolved gases such as air as a consequence of the mode of theirpreparation or purification. Removal of these gases is frequentlynecessary to carry out the aforementioned industrial processessatisfactorily, but this is often difficult to accomplish as, forexample, in the case of foamy or viscous liquids. In the manufacture ofphotographic products, in particular, it is essential to remove bubblesof air from the prepared solutions, which can be foamy as well asviscous, and to prevent subsequent formation of new bubbles in thesolutions in order to ensure a uniform, smooth coating of the requiredthickness.

U.S. Pat. No. 2,634,907 discloses a centrifuging process and apparatusfor continuously removing gas in which the liquid to be deaerated is fedinto the centrifugal zone of a cylindrical rotor and the deaeratedliquid is removed through a liquid seal into a substantially gas-freecollecting zone. The level of liquid within the rotor chamber ispreferably controlled by the size of the opening in an orifice platevalve located at the outlet.

U.S. Pat. No. 2,785,765 discloses a centrifuging apparatus for degassingliquids, which apparatus contains, in an air-tight housing, a generallydome-shaped rotatable member under a condition of partial vacuum intowhich liquid is fed through a "buttering ring" and from which thetreated liquid is removed by a scoop that is a hollow tube of curvedcontour.

U.S. Pat. No. 3,228,595 discloses a centrifuge discharge means that is askimmer having an elongate body and a leading end that is divided intoan upper opening and a lower opening. The skimmer in operation is onlypartly immersed in the liquid to reduce the foaming tendency of theliquid.

Some fluids which are required to be degassed are not solutions butrather suspensions or emulsions. U.S. Pat. No. 3,996,117 discloses adischarge device for removing a fluid from a fluid processing apparatus,which device causes the fluid to flow around its converging sidesurfaces in a streamlined flow pattern and causes sufficient sub-surfaceturbulence to keep particles originally dispersed throughout the fluidfrom settling toward the bottom, which device further, by means of oneor more inlet ports in its side surfaces, enables fluid to flow to theoutlet of the fluid processing apparatus.

A vacuum degassing centrifuge apparatus ideally operates in a steadystate condition in which the level of degassing liquid in the annulartrough of the centrifuge bowl is maintained at a substantially constantlevel. This condition could theoretically be attained by using twoperfectly matched pumps that produce exactly equal inlet and outletrates. In practice this situation is virtually impossible to achieve,particularly when one takes into account other potential disruptions inthis theoretical equilibrium condition, for example, when it becomesnecessary to change a filter in the system. Under these circumstancesthere is a need for an additional means of controlling the level ofliquid in the centrifuge bowl.

One such means of liquid level control that has been employed utilizessensing probes located within the trough of the centrifuge bowl. Inparticular, conductivity sensors have been mounted on the leading edgeof the centrifuge discharge device disclosed in U.S. Pat. No. 3,966,117.If a conductivity sensor immersed in the collected liquid becomesuncovered, a valve is opened to allow more liquid to enter the bowl. Asthe sensor again becomes covered, the valve is closed to stop theinflow.

The above described method of sensing and controlling the level ofliquid in the centrifuge bowl is disadvantageous in that the operationof the valve is abrupt, leading to significant pressure fluctuations.The larger variations in pressure give rise to a hydraulic hammeringeffect that damages filters in the system. Smaller pressure changes, onthe other hand, cause flow spikes in the degassed liquid exiting theapparatus; if that liquid is, for example, a photographic emulsion to becoated, an uneven, defective coating is the result.

BRIEF SUMMARY OF THE INVENTION

The present invention provides an apparatus for degassing a continuouslyflowing liquid feed stream which comprises an air-tight housing, inwhich is mounted for rotation about an axis a centrifuge bowl having acircular cross-section. The axis of rotation extends through the centerof and perpendicular to the cross-section of the bowl. The bowl has aninner surface for the spreading of centrifugally impelled liquid in athin layer and an annular peripheral trough for retaining centrifugallyimpelled liquid as the bowl rotates. The apparatus in accordance withthe invention further comprises means for feeding the aforementionedliquid stream into the centrifuge bowl and means for withdrawing fromthe bowl a stream of degassed liquid which includes a tube extendingfrom an exit line outside of the apparatus housing to an open end ofelongated cross section which is positioned within the annular trough ofthe bowl and which is immersible in the degassed liquid. The apparatusin accordance with the invention further comprises a means for measuringthe pressure of the degassed liquid stream exiting the centrifuge bowlthrough the aforementioned tube, a means responsive to theaforementioned means of measuring the pressure of degassed liquid inorder to regulate the flow of the liquid feed stream into the centrifugebowl so as to maintain a substantially constant level of liquid in theannular trough of the bowl, a means of withdrawing gas from theapparatus housing and maintaining therein a sub-atmospheric pressure,and a means for regulating the flow of the degassed liquid stream.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagrammatic representation of the apparatus of theinvention.

FIG. 2 is a front view, with a part broken away, of the centrifuge bowl,tube, and housing of an embodiment of the invention.

FIG. 3 is an end cross-sectional view of the apparatus of FIG. 2.

FIG. 4 is a detailed side view of the tube and splash guard as employedin a preferred embodiment of the invention.

FIG. 5 is a detailed end view of the same portion of the apparatus as inFIG. 4.

FIGS. 6 and 7 are detailed end and side views, respectively of the tubedepicted in FIGS. 4 and 5.

FIG. 8 is a view, similar to that of FIG. 2, showing the position of thetube in the centrifuge bowl and the orientation of the open end of thetube in a preferred embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention overcomes the aforementioned disadvantages ofpreviously known methods of sensing and controlling the liquid level inthe centrifuge bowl by measuring the pressure of the degassed liquid inthe exit line outside of the degassing apparatus, which increases withincreasing liquid level in the bowl, and using that measured pressure toregulate the inflow of liquid so as to maintain the level of liquid inthe centrifuge bowl at a substantially constant level.

FIG. 1 is a diagrammatic representation of the vacuum degassingapparatus of the invention. The liquid to be degassed is fed by inletline 1 into the center of the inner surface 2 of a centrifuge bowl 3,which is located within an air-tight housing 4 and is mounted forrotation about an axis 5. In the representation of FIG. 1 the bowl ismounted for rotation about a horizontal axis.

FIG. 1 also depicts the annular peripheral trough 6 of the centrifugebowl, which retains the centrifugally impelled liquid as the bowlrotates; a tube 7 with an open end 8 is positioned within the trough ofthe bowl and connected to exit line 9. A means for measuring thepressure of the degassed liquid, pressure sensor 10, is connected with ameans for regulating the flow of the liquid feed stream,,control element11 and pump 12, thereby maintaining a substantially constant level ofliquid in the trough of the centrifuge bowl. Gas is withdrawn from theair-tight housing through the vacuum line 13 by means of the vacuumcontrol 14 and the vacuum source 15. The metering pump 16 regulates theflow of the degassed liquid.

In FIG. 1 a variable speed positive displacement pump 12 is depicted forfeeding the liquid stream into the bowl. Other suitable means include acentrifuge pump, a gear pump, a valve, a pressurized supply tank, agravity feed means, or a combination thereof.

In accordance with the invention, the pressure of the degassed liquidstream in line 9 is measured by a pressure sensor 10. The latter can beany of a number of well known devices for measuring the pressure of aliquid stream, such as an electrical transducer. Also suitable is apneumatic transducer, a hydraulic gauge, or a combination of suchdevices.

The means responsive to the pressure sensor 10, the control element 11,can be a computer, a programmable logic-controller, a proportionalintegral derivative controller, or a combination thereof.

In accordance with the invention, means for withdrawing gas from theair-tight housing include the vacuum control 14, which can be a valve, acomputer, a programmable logic controller, or a combination thereof, anda vacuum source 15, which is preferably a water ring pump. Othersuitable devices include a diffusion pump, a mechanical pump, anaspirator, or a combination thereof.

In FIG. 1 a positive displacement pump is depicted as the metering pump16. Other suitable means for regulating the flow of the degassed liquidinclude a centrifuge pump, a gear pump, a valve, or a combination ofsuch devices.

FIGS. 2 and 3 depict in detail an embodiment of the invention, showingthe inlet line 1 which feeds the liquid onto the inner surface 2 of thecentrifuge bowl 3, which is located within the airtight housing 4 and ismounted for rotation on the axis 5. A splash plate 17 is mounted overthe mouth of the bowl. Also shown are the annular peripheral trough 6and the tube 7 with its open end 8 positioned within the trough andconnected to the exit line 9. Affixed to the tube is a splash guard 18.

In FIGS. 4, 5 and 6 are depicted a preferred embodiment of the tube 7,in which its open end 8 has been shaped to produce an elongatecross-section which has a ratio of the long dimension to the shortdimension of about 10:1. Also shown is the splash guard 18.

FIG. 7 provides further detail of the chamfered open end 8 of the tube7, that has been formed into an elongate shape with a sharp streamlinededge.

In FIG. 8 is depicted a preferred embodiment of the invention, in whichthe open end 8' of the tube 7' is substantially perpendicular to thedirection of liquid movement and is preferably positioned, for thepurpose of reducing splashing, at an angle of from about 90° to about45°, as measured from the perpendicular from the axis to the lowermostpoint of the centrifuge bowl.

The shape of the open end 8 of the tube 7 affects the fluctuations inthe pressure of the degassed liquid exiting the apparatus of theinvention. This is demonstrated by the tests that are described in thefollowing example.

Example

A series of four tubes was made from pieces of nominal 1/2" I. D.stainless steel tubing which had been bent into the shape depicted inFIG. 3. The inner edge of the open end of the tube was smoothed to asharp streamlined edge, and the ends of the tubes were crimped toproduce elongate cross-sections of various dimensions. The crimp in eachcase was about 2.54 cm (1 in) in length, but it was separatelydetermined that variations in the length of the crimp did notsignificantly affect pressure pulsations. The tubes thus produced hadelongate cross-sections in which the ratios of long dimension to shortdimension were 5:1, 10:1, and 20:1, respectively. In addition, anuncrimped tube with a round open end was included in the test. The tubeswere positioned inside the centrifuge bowl of the vacuum degasser testapparatus, and a gelatin solution with a viscosity of 40 cp was fed intothe apparatus at a flow rate of about 18 kg/min. The tubes in thedegasser were connected by a 3.6-m length of flexible tubing to a DataInstruments AB™ pressure transducer with a range of 0-100 psi (0-689.5KPa). Signals from the transducer were fed to a Keithley dataacquisition system (from Keithley/Asyst Co.) which had been configuredto collect pressure readings at a frequency of 25 Hz over a span of 12seconds.

The following table lists the standard deviations in the outlet liquidpressure that were determined for the four tubes included in the test:

    ______________________________________                                                        Outlet liquid pressure                                                        standard deviation                                            Cross-section of tube                                                                           psi      KPa                                                ______________________________________                                        round             .737     5.08                                               elongate (5:1)    .542     3.74                                               elongate (10:1)   .344     2.37                                               elongate (20:1)   .252     1.74                                               ______________________________________                                    

These results show the significant reduction in liquid pressurefluctuation that resulted from changing the cross-section of the tubeopening from round to elongate with a dimensional ratio of 10:1. Afurther lowering of pressure variations was observed with the tubehaving an opening with a 20:1 elongate cross-section, but thisimprovement was accompanied by a further reduction in flow rate, so the10:1 elongate tube opening was preferred.

The pressure of degassed liquid exiting the centrifuge bowl that is usedto regulate the flow of the liquid stream and maintain a substantiallyconstant liquid level in the bowl in accordance with the invention isdependent on the aforementioned liquid level, the predetermined speed ofrotation of the bowl, and, to a lesser extent, the viscosity of theliquid. Adjustment of the pressure set point enables the apparatus ofthe invention to be employed to degas solutions with viscosities of fromless than 1 to about 250 cp.

Measuring the pressure of degassed liquid in the exit line outside ofthe degassing apparatus and using that measured pressure in accordancewith the invention to regulate the inflow of liquid by means of avariable speed positive displacement pump so as to maintain the level ofliquid in the centrifuge bowl at a substantially constant level enabledthe fluctuations in the pressure of exiting liquid to be minimized andmaintained in the range from about 1.4 to about 2.8 KPa. This was a verysignificant improvement over the results obtained when the inflow ofliquid in the centrifuge bowl was controlled by a valve that wasregulated by a conductivity sensor located within the bowl, under whichcircumstances liquid pressure fluctuations from about 35 to about 50 KPawere observed.

An additional advantage of using pressure measurements in accordancewith the invention rather than conductivity measurements to maintain asubstantially constant level of liquid in the bowl is that the apparatusof the invention can be used to degas liquids of low conductivity, forexample, organic solvents.

In accordance with the invention, a sub-atmospheric pressure ismaintained within the air-tight housing of the vacuum degassingapparatus. In a preferred embodiment, a water ring pump is used toproduce a pressure of about 10 KPa.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

We claim:
 1. A process for degassing a continuously flowing liquidstream which comprises:feeding said liquid stream into a centrifuge bowlhaving an inner surface for the spreading of centrifugally impelledliquid in a thin layer and an annular peripheral trough for retainingcentrifugally impelled liquid, said bowl being mounted for rotation androtating within an air-tight housing that is maintained at asub-atmospheric pressure; forming on the inner surface of said bowl athin layer of centrifugally impelled liquid to remove entrained gas;collecting degassed liquid in the annular peripheral trough of saidbowl; withdrawing said degassed liquid from the annular peripheraltrough of said bowl by means of a tube connected to an exit line andhaving an open end which is positioned within said annular trough;measuring in said exit line the pressure of liquid exiting said bowlthrough the open end of said tube; and regulating the flow of liquidinto said bowl in response to the measurement of said pressure tomaintain a substantially constant level of liquid in the peripheraltrough of said bowl.
 2. A process according to claim 1 wherein saidliquid is a photographic gelatin emulsion.
 3. A process according toclaim 1 wherein said liquid stream is fed into said bowl by means of apositive displacement pump.
 4. A process according to claim 3 whereinthe level of liquid in the peripheral trough of said bowl is maintainedby controlling the speed of rotation of said positive displacement pumpin response to said measurement of pressure.
 5. A process according toclaim 1 wherein said open end of said tube has an elongatecross-section.
 6. A process according to claim 1 wherein the pressure ofliquid exiting said bowl is measured by a pressure transducer.